FIG. 1 is a perspective diagram for explaining the principal part of an ink jet head that uses electrostatic force, to which the present invention is to be applied, and FIG. 2 is a cross-sectional view of the principal part, showing the structure of an actuator of the ink jet head that is shown in FIG. 1 (outline cross-sectional view in a direction of the longer edge of a vibrating plate: a sectional view of FIG. 1 viewed at the line II—II). These figures show an electrode board 10 that includes an electrode 11, a liquid chamber board 20 that includes a vibrating plate 21 formed when an ink chamber 22 is carved, and a common ink chamber 23 that supplies ink to each ink chamber, and a nozzle board 30 that includes a nozzle 31 that discharges the ink in the liquid chamber 22. The electrode board 10, the vibrating board (liquid chamber board) 20, and the nozzle board 30 are laminated. (Although FIG. 1 and FIG. 2 show an example of a side shooter structure, an end shooter structure may be used.) In the vibrating board 20, which serves as a part of the ink chamber 22 and as a part of the common liquid chamber 23, the ink chamber 22 that is connected with the nozzle 31, and the vibrating plate 21 that is conductive and made thin in order to attain low rigidity such that it is flexible are provided.
The electrode board 10 includes an individual electrode 11 that is installed facing the vibrating plate 21 with a predetermined gap from the vibrating plate outside the ink chamber. Although a protection film 12 for preventing a short circuit etc. with the vibrating plate 21 is formed on the individual electrode 11, a protection film may also be formed in the back (on the side that faces the electrode) of the vibrating plate 21, if desired.
As shown in FIG. 1, a plurality of actuators as shown in FIG. 2 are installed in the electrostatic ink jet head to which the present invention is to be applied, and an ink drop is discharged from each of the actuators.
In FIG. 1 and FIG. 2, when voltage is applied between the vibrating plate 21 and the individual electrode 11, the vibrating plate 21 is displaced toward the electrode 11 by electrostatic force. When the voltage is removed, the vibrating plate 21 returns to the previous position, that is, the position before the voltage was applied. The electrostatic ink jet head uses this mechanical behavior in response to the electrostatic force of the vibrating plate 21 as the ink discharging force of an ink jet. In each actuator, a space 40 that is formed by the electrode board 10 and the vibrating board 20 is called a gap chamber, and a space that is a part of the gap chamber, formed by the vibrating plate and the electrode board is called a vibrating chamber.
In the electrostatic ink jet head as mentioned above, the vibrating plate 21 is made thin in order that a driving voltage to generate the electrostatic force between the vibrating plate 21 and the individual electrode 11, which displaces the vibrating plate 21, can be low, the voltage being applied between the vibrating plate 21 and the individual electrode 11 of the electrostatic actuator. A thin vibrating plate requires a lower driving voltage, however, rigidity of the vibrating plate becomes low. Where the rigidity is low, presence of air (or other gas) in the vibrating chamber and the gap chamber greatly affects the behavior of the vibrating plate. For example, when the vibrating plate 21 approaches the electrode 11, compression resistance of the air causes the voltage required to make the vibrating plate 21 contact the electrode 11 (the voltage is hereafter called the contact voltage) to become large in a dynamic situation, as compared with a static situation. To this problem, certain measures have been developed. For example, a Japan Provisional Publication No. 7-299908 has been published, wherein a gap chamber is provided in addition to the vibrating chamber such that the air escapes when the vibrating plate is displaced toward the electrode'side, and the compression resistance of the air is prevented.
The present invention is made for the purpose of coping with another significant problem, as explained below, resulting from the presence of the air as mentioned above.
Sections (A) through (D) of FIG. 3 show an outline structure of the principal part of the electrostatic ink jet head, and are for explaining the problem to be solved by the present invention. The sections (A) and (B) of FIG. 3 show a range of actual displacement (L) of the vibrating plate when the driving frequency is low. The sections (C) and (D) of FIG. 3 show the range of actual displacement (1) of the vibrating plate when the driving frequency is high.
The sections (A) and (C) of FIG. 3 show sectional views in the longer edge direction of the vibrating plate. The sections (B) and (D) of FIG. 3 show sectional views in the shorter edge direction of the vibrating plate. The vibrating plate of the conventional electrostatic ink jet head is required to vibrate dynamically in a range of up to 10 kHz. Since the space between the vibrating plate and the electrode is narrow, wherein the vibrating plate vibrates at a high speed, as mentioned above, the vibrating plate 21 receives compression resistance of air during the period of movement toward the electrode 11. A portion of the air escapes to outside of the vibrating chamber 40 (as an arrow shows), according to the vibration. This phenomenon is called a squeezing effect. Then, when the voltage is removed and the vibrating plate 21 separates from the electrode 11, the inside of the vibrating chamber 40 is reduced to a state of negative pressure compared to the atmosphere. Due to this, the position to which the vibrating plate 21 returns is a position closer to the electrode 11 than the original position. Here, the amount of the air that is pushed out from the vibrating chamber 40 in a certain unit period increases as the proportion of the period during which the vibrating plate 21 contacts the electrode 11 increases. That is, the higher the driving frequency is, and the wider the driving electric pulse width is, the larger the amount of the air pushed out from the vibrating chamber to the outside is, and the larger the negative pressure of the vibrating chamber is, making the position to which the vibrating plate returns when the electric pulse is turned off to be closer to the electrode.
FIG. 9 shows an example (contacting period dependency of a parallel gap). The figure shows a result of measurement of vibrating displacement at the center position in the shorter edge direction of the vibrating plate of the actuator, the measurement being performed by a laser Doppler vibrograph. The vertical axis represents a displacement amount δ, and the horizontal axis represents magnitude of the driving voltage, where the waveform of the driving voltage is rectangular. The δ-V characteristic is expressed with the driving frequency serving as a parameter. There are areas where a displacement amount saturates at a certain voltage. The saturated displacement amount is called the contact displacement amount.
In reference to FIG. 9, the higher the driving frequency is, the larger the amount of the air that is pushed out from and cannot return to the vibrating chamber is. For this reason, as the sections (C) and (D) of FIG. 3 indicate, the vibrating plate 21 vibrates closer to the electrode 11, making the distance between the electrode and the vibrating plate substantially short, causing the contact voltage to drop. Thus, there is a phenomenon that does not cause a problem when the driving frequency is low, but becomes a problem when the frequency is made higher, and when the pulse width of the driving voltage is made wider.
Although the above subject is not a problem in a conventional electrostatic ink jet head that operates at most at about 10 kHz, it is a problem that should be solved in a head that serves a future high-speed printer. However, no countermeasure to this problem has been proposed.
Here, the problem is applicable to contact driving in which the vibrating plate contacts the electrode. In the case of non-contact driving in which it does not contact, the frequency dependent problem mentioned above does not arise or does not pose a problem.